Usually, an electronic device or, more specifically, a portable electronic device such as a portable media player (PMP), a portable media center (PMC), a MP4 player, a personal digital assistant, a cell phone, a digital camera, etc. may be powered by more than one power sources. Multiple power supply is considered when a load (e.g. a battery, an integrated circuit, or a system in the electronic device) demands a high current, voltage or power input that a single power source is incapable to provide. An electronic device receiving power from an AC adapter and a USB power source concurrently is a typical example of the multiple power supply. The “USB power source” herein refers to a power source that delivers power through a USB interface to the electronic device.
Furthermore, in order to meet the demands of the electrical requirements (e.g. voltage, current, or power requirement) from the load, a power converter is commonly employed by most of the electronic device to regulate the raw voltage, current or power delivered from the power source and to generate a stable and desirable output to the load. The power converter is usually coupled between the power sources and the load. There are two different types of power converter. One is switched mode, where the electronic device is charged through some periodically on and off switches. The other one is linear mode, where the device is charged through a variable resistor. However, a switched mode power converter with multiple power inputs is rarely seen. Usually, a linear converter is adopted in multiple power supply application.
FIG. 1 shows a prior art power supply architecture 100 using a linear power converter with multiple power inputs. As illustrated, POWER SOURCE 1, POWER SOURCE 2, . . . and POWER SOURCE N simultaneously provide power to a load or system. A linear mode power converter 102 with N power controlling devices (i.e. variable resistors) coupled to respective power sources and a common node VMAX is provided. By controlling and changing the resistance of each resistor, the total amount of power delivered to the load can be adjusted to a required value. However, the shortcoming of this solution is obvious. A large voltage difference between different power sources will result in a huge power loss and unfavorable heat producing on the power controlling device. For example, suppose a situation when a 5.0V USB power source with 500 mA current capacity and a 3.7V battery both concurrently supply the power to the load. When the load current requires less than 500 mA, only USB is needed and the load voltage is 5V. When the load current increases and requires more than 500 mA, the battery enters into operation to supply the additional current while the USB current is limited to 500 mA and the load voltage drops to 3.7V. In this case, there is a 1.3V voltage drop across the associated power controlling device connected to the USB power source and the power loss across the power controlling device is 1.3V×500 mA=0.65 W. As a result, undesirable heat is produced. However, some of the linear mode power converters are improved with a thermal regulation technology. When the temperature over the power controlling device rises to a certain degree, the charge current is reduced. In this way, the chip is protected from over-heating. But the thermal regulation technology may cause another shortcoming, that is, the charge current becomes small.
Compared with the linear technology, a switch mode power converter works more efficiently due to the switching characteristics of switching elements (e.g. MOS transistor or diode, etc.) in the converter. But a conventional switched mode power converter is not capable of enabling multiple power supply to provide power concurrently. Generally, only one of the multiple power sources is selected to provide power to the load at one time. FIG. 2 shows another prior art power supply architecture 200 using a switch mode power converter with power selecting functionality. As illustrated, a plurality of switches SW1 201, SW2 202, . . . , and SWN 203 are provided to select one of the plurality of power sources POWER SOURCE 1, POWER SOURCE 2, . . . , and POWER SOURCE N to deliver power to a load or a system via a switched mode power converter 204. Usually, each power source is assigned a priority. The electronic device first checks the power source with the highest priority. If the power source with the highest priority is available, this power source is then selected to supply the power to the load. If the power source with the highest priority is not available, a next power source with lower priority is checked. It is known that no matter which power source is selected; only one switch is turned on or closed at a time. The reason is that if more than one switch is turned on, the associated power sources will short each other. As a result, the problem with this method is that the power sources are not fully utilized.
FIG. 3 illustrates yet another prior art power supply architecture 300 with a charger and battery to supply power to a load. As illustrated, a charger 302 (also known as an external power source) and a battery 304 are coupled together to a load. In this case, the charger, or the external power source 302, is only used to charge the battery 304 and the load gets power only directly from the battery 304. The charger 302 can receive power from one or more external power sources POWER SOURCE 1, POWER SOURCE 2, . . . , and POWER SOURCE N as illustrated in FIG. 3. The problem with this architecture is that the load voltage, current, or power is latched to the battery's voltage, current, or power. When the battery 304 is discharged below the minimum voltage required by the load, the load or the system cannot start immediately even when external power source 302 is available and have enough current capacity.
Therefore, it is to an improved and efficient power supply system that overcomes the above-mentioned shortcomings of the several types of conventional power supply systems and emulates an equivalent effect of multiple power supply to a load via a switched mode power converter that the present invention is primarily directed.